In the application of the phase cut dimming power supply, in order to produce good dimming effect, it is necessary for the phase cut dimming power supply to provide the transistor in the phase cut dimmer with a current to maintain a conducting state. Such a current should be stable, but not excessive, as excessive current requires more power consumption and reduces the overall conversion efficiency of the power supply.
At present, what is used most is to connect an X capacitance (or thin-film capacitor) and a bleeder resistance to the input terminal of the power supply or connect an X capacitance (or thin-film capacitor) and then serial in a DM (differential mode) inductor in the front of such an X capacitance, to provide holding current for the dimmer. Such a method has great power consumption in the bleeder resistance and DM inductor and great heat radiation. The use of such a circuit has a very low efficiency. Moreover, as the temperature of the bleeder resistance and DM inductor increases greatly, it will also affect the reliability of the whole machine. It is also more difficult to control and adjust the size of the holding current with such a method. Meanwhile, it is very easy to make the dimmer vibrate, resulting in noises.
To sum up, in the existing technologies, the phase cut dimmer holding current control circuit has a high power consumption, high temperature rise and is hard to adjust but prone to cause noise in the dimmer among other defects.